Switched mode power supplies can be used to create a direct current (DC) voltage from an alternating current (AC) voltage by switching current through an energy storage element such as a transformer. The duty cycle of the switching is controlled to regulate the output voltage to a desired level. Flyback converters are a type of switched mode power supplies that are common in AC-to-DC voltage applications. A flyback converter is based on a flyback transformer that alternately builds up flux in the magnetic core and transfers energy to the output. When current is switched through the primary winding, the primary current in the transformer increases, storing energy within the transformer. When the switch is opened, the primary current in the transformer drops, inducing a voltage on the secondary winding. The secondary winding supplies current into the load. A controller varies the on- and off-times of a primary switch in series with the primary winding to regulate the output voltage to a desired level.
Flyback converters use a rectifier connected to the secondary winding to prevent the reverse flow of current through the secondary winding. The rectifier can take two forms. A passive rectifier, such as a diode, can be placed in series with the secondary winding to prevent reverse current flow. However the diode cannot properly prevent reverse current flow if the output power supply voltage exceeds the breakdown voltage of the diode. Moreover the diode causes a forward voltage drop when conductive, decreasing the efficiency of the converter. To solve these problems, another form of rectifier known as a synchronous rectifier is often used. A synchronous rectifier includes an active switch, usually an N-channel metal-oxide-semiconductor field effect transistor (MOSFET), connected in series with the secondary winding along with a controller that makes the transistor conductive at the appropriate time. Because the transistor can be biased fully on, synchronous rectifiers are generally more efficient than passive rectifiers.
However when the drain voltage of the synchronous rectifier (SR) transistor rises rapidly due to switching at power up, the gate voltage of the SR transistor could also rise quickly prior to the power up of the SR controller due to capacitive coupling between the drain and gate and between the gate and the source. At power up, the controller cannot keep the gate voltage low because it is not powered up yet. If the voltage on the gate of the SR transistor rises too much, it could cause the SR transistor to become conductive, creating undesirable shoot-through currents on the secondary side and causing potential damage to the system.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.